Three-dimensional scan converter for ultrasound imaging

ABSTRACT

A method and apparatus of a three-dimensional scan converter system for generating real-time C-scan and transparent images in addition to two-dimensional B-scan images in an ultrasound imaging system. The present invention has means for ultrasonically scanning a subject to obtain image data, an image memory means for storing the image dam, a two-dimensional scan converter means for converting the stored image data into a three-dimensional cubic data matrix, and a means for displaying the image.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed generally to a three-dimensional scanconverter (3D SC) for use in 3D ultrasound imaging, and specifically toa 3D SC apparatus that converts the input 3D data in linear-linear,sector-linear, sector-sector and linear-sector scan formats to a cubicrasterized data matrix.

2. Background of the Invention

Two-dimensional ultrasound imaging has been widely used in cardiology,radiology and other clinical diagnostic areas for more than thirtyyears, since J. J. Wild and J. M. Reid published their first paper ofultrasound medical B-scan imaging in Science in 1952. As an example,two-dimensional echocardiography offered considerable advantages overM-mode echocardiography because of the ability to provide real-timetomographic images of the heart, and it extended the ability ofpractitioners to make complex diagnostic decisions. In the beginning,there were questions by some whether two-dimensional approaches wereworth the tithe and expense. Continued experience with these methodsprovided an opportunity for clinicians to ask new questions. Althoughsimilar questions have been directed to three-dimensional ultrasoundimaging for years, three-dimensional echography appears to be adesirable goal as it would likely provide a method for deriving newanatomical and functional indices of the human heart and other organs.

Early efforts on the investigation of three-dimensional ultrasoundimaging can be tracked back to later 50's to earlier 70's. But theresults on 3-D medical ultrasound imaging, such as, ultrasoundtransmission tomography and holography, were unfortunately disappointingbecause of poor image quality caused by the diffraction effect, soundspeed variations in tissues and the penetration limitation of sound insome tissues and organs, such as bone and lung. Data acquisition andimage reconstruction speed was too slow to provide any meaningful andpractical clinical use. Even though the investigation never stopped, thepublic interest in 3-D ultrasound imaging kept a low profile for a longtime.

Following recent advances in computer technology and high-speed digitalelectronics interest in three-dimensional medical ultrasound imaging hasgradually recovered. High speed 3-D imaging systems with heavy parallelprocessing have been built to supply as fast as 15 volume-per-secondreal-time imaging frame rate, even though the image quality is stillvery poor compared with the image standard for the up-to-datetwo-dimensional B-scan. The clinical applications of ultrasoundthree-dimensional imaging have been reported in many areas, fromcardiology to ophthalmology.

The benefits to be obtained from current 3-D ultrasound imaging systemare still debated because of the low frame rate and low signal-to-noiseratio. Both these existing problems could be overcome in the future withthe advance of technology. It is clear that there are strong, andgradually increasing clinical needs for this type of imaging modality.Vast literature has been published in recent years on the clinicalapplications of the ultrasound three-dimensional imaging technique.

In cardiology, despite the obvious advantages of two-dimensionalechocardiography methods over M-mode, serious limitations remain. It isimportant to obtain a short axis B-mode sector scan of the leftventricle of the heart. But it is limited to some extent by the acousticwindows offered by the rib cage and the lungs. With a three dimensionalimaging system, a C-scan plane can be walked vertically through thesector, providing short axis view at any desired level, from the apex,through the center of the ventricle, through to the level of the mitralvalve and beyond. In some patients, such a variety of scans would not beavailable to the physician with a standard two-dimensional system,because of the restrictions of the acoustic window.

Cardiac structures are spatially complex and a mental picture of theheart must be acquired from a series of two-dimensional interrogations.For example, calculation of ventricular volume by echocardiography mustbe performed based upon complex geometric assumptions such as whether aventricle is elliptical or not. In the setting of a severe regionalwall-motion abnormality, the ventricle may not conform to any geometricassumption and the resultant quantitative volume information is,therefore, limited.

Likewise, quantitative assessment of wall motion data derived fromtwo-dimensional echocardiography is subject to limitations imposed bythe complex spatial forward and rotational movements of the heartbetween diasrole and systole. Almost all such computer based models are,therefore, inherently limited because it becomes spatially impossible todetermine the same geometric center of the ventricle between diastoleand systole for reliable determination of wall-motion indices.

In addition, the normal breast is characterized by a well-orderedspatial organization of the connective and glandular tissues. 3-Dreconstruction allows surface analysis of the tumor smooth envelope, andis clearly distinct from the normal parenchyma. The adenocarcinoma hasan irregular, jagged envelope, with poor limitations from thesurrounding tissue. 3-D will lead to significant increases inspecificity and sensitivity for breast tumor diagnosis with ultrasoundand to better comprehension of cancer-dystrophy relationships. 3-D willalso lead to progress in antenatal diagnosis through spatialvisualization of fetal organs, and allow the development of newdiagnostic and therapeutic procedures in utero.

Interventional Cardiology has grown very rapidly in recent years.Ultrasound imaging catheter has been used for thrombus and stenosisdiagnostics during PTCA procedures. Because of lacking three-dimensionalspatial scanning capability, current image interpretation is relying onphysician's experience and still of manual catheter manipulation. Inrecent years, several prototype 3-D imaging catheters have beenpresented. By off-line processing, series 2-D image sections scanned indifferent depths are stacked together to reconstruct a three-dimensionalcoronary artery image. This 3-D image not only speeds the diagnosticprocess, but also increases the diagnostic efficacy by providingphysicians the ability to view the diseased artery segment in alldirections most of which are not accessible with conventional diagnostictechniques. Some of the prototypes even can open and flat the arterysegment to let physicians inspect the inner wall of the artery which hasvery important information for the diagnosis.

During the past two decades, medicine has benefited from a trend towardminimally invasive procedures. Miniature surgical devices are beingdeveloped that can be introduced through small incisions to perform manyelective surgical procedures. Many of these procedures will requireultrasonic imaging techniques for guidance. In some situations,three-dimensional scanning capability, with automatic localization anddisplay of the scan plane containing an interventional device, wouldhelp by eliminating the need to manually track the interventionaldevice.

The three-dimensional ultrasound imaging and related tissuecharacterization methods have been applied fruitfully in a number ofmedical applications, including evaluation of intraocular tumors,cancers of the liver, clots and thrombi, skin lesions and prostatetissue.

More clinical applications of the three-dimensional ultrasound imagingwill be discovered after the imaging system is commercially availableand more physicians get familiar with this imaging technology.

Ultrasound's low cost, noninvasive, and repeatable way of capturingdynamic images have led to the widespread success of two-dimensionalB-scan imaging. These benefits should apply equally forthree-dimensional ultrasound imaging. Current 2-D imaging system canobtain sector, linear and other formats of real-time images in a framerate greater than 30 frames per second. Image detail resolution has beenpushed almost to the ultrasound diffraction resolution which is theultimate limit in this type of imaging methodology. The contrastresolution has reached the negative 70 dB level. There is still a hugegap between the current two-dimensional image quality and thethree-dimensional image quality which can be realistically achieved inthe near future.

One major technical obstacle is how to convert 3D ultrasound scanformats, such as sector-sector, to regularized raster cubic data matrixfor image processing and CRT display. Due to the huge amount of datareceived from 3D scan, the 3D scan conversion could take a long time ifusing conventional computation technique. This is not acceptable forreal-time clinical requirements.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an apparatus of thetype initially cited to convert the three-dimensional ultrasonicallyscanned image data into a cubic data matrix.

It is an object of the present invention to provide a real-time 3D thegoal of this invention is to provide a real-time 3D scan converter whichuses 2D scan converters and image memories in most of existingultrasound imaging systems to accomplish the 3D scan conversion by twosets of 2D scan conversion in two orthogonal directions. This designsignificantly simplifies the conversion process and greatly reduces thecomputation need.

The above object is achieved in accordance with the principles of thepresent invention in an apparatus having a three-dimensional scanconverter system for generating real-time C-scan and transparent imagesin addition to two-dimensional B-scan images in an ultrasound imagingsystem, comprising means for ultrasonically scanning a subject to obtainreal-time image data representing an image of said subject, saidultrasonic scanning means further comprises transducers and athree-dimensional position control means having a position encoder forindicating current transducer position, image memory means for storingimage data representing an image of a subject, two-dimensional scanconverter means for converting the stored image data intothree-dimensionally raster formatted stored image data in real-time,image processing means for operating on said three-dimensionallyformatted stored image data for successive display of said orthogonalmultiplanar images, system software and a CPU, having one or morelook-up tables containing scan converter parameters corresponding tosaid transducer position therein, for downloading said parameters inreal-time to said scan converter means, a raster processing means forcompounding said transparent image, and display means for displayingsaid three-dimensional images.

The three-dimensional scan converter (3D SC) of the present inventionprovides real-time C-scan and transparent images in addition to theregular 2D B-scan images. In another embodiment of the three-dimensionalscan converter of the present invention, the conversion is done onlyafter the scan. The image data is saved in the image memory (IM) duringthe scan. This yields better interpolation quality and the imageacquisition control is simpler than that of the first embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention shall be set forth in greater detail by way of examplewith reference to the figures.

FIG. 1a shows the 3D scan converter within the 3D ultrasound imagingsystem of the present invention.

FIG. 1 shows the sector-sector scan format of the present invention.

FIG. 2 shows the sample data storage vector by vector in the imagememory (IM) of a sample value in 3D space location.

FIG. 3 shows a sample data set converted to a cubic voxel matrix of thepresent invention.

FIG. 4 shows a representation of how 2D images are acquired withconstant tangent-angle intervals between the scan plane of the presentinvention.

FIG. 5 shows a graphic representation of X_(IncrL) as defined in thepresent invention.

FIG. 6 shows the image buffer 1 and buffer 2 having C-scan images atdepths i and i+D of the present invention.

FIG. 7 shows two C-scan image depths of i and i+D of the presentinvention.

FIG. 8 shows a flowchart of first embodiment of a scan converter of thepresent invention.

FIG. 9 shows a flowchart of the second embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before a description of the preferred embodiments, the following termsas used herein have the following definitions. Multiplanar images arethree orthogonal images in 2D planes, obtained from 3D scan volume datasets. One of the multiplanar images which is parallel with thetransducer surface has been called C-scan x(i), in the literature. Cmeans Constant depth. Also, a transparent image which is known otherwiseas a Projection Image, is the result of spatial compounding of 2D imagesfrom a 3D data set. In grey-scale imaging (GSI) it looks similar to anX-ray image. The 3D ultrasound scan format is defined by its 2D scanformat and the third dimension scan format. For example, a curved-linearformat means that the curved array moves linearly in the transducerelevation direction. There are a number of other scan formats, such aslinear-linear, sector-linear, sector-sector, linear-sector, andlinear-circular. FIG. 1 shows the detail of sector-sector format as anexample for further discussion. All scan formats are acquired in therestricted mode which means that the scan format and the 2D slicelocations are defined before the scan begins.

FIG. 1a shows the system flowchart of the present invention. Real-time2D images are acquired by the array transducer which has a 3D positioncontrol attachment. The transducer is moving in the φ directionindicated in the FIG. 1. The transducer movement is provided by a motoror the operator's hand. In the 3D attachment, there is a positionencoder which indicates the current transducer position. The number of2D slices to form the 3D volume is fixed. During real-time scanning, one2D slice, and only one 2D slice, for each transducer position is sent tothe Image Memory (IM), as well as to the Scan Converter. For eachtransducer position, there is one set of SC parameters which isdown-loaded into the SC from the look-up tables (LUTs) in the systemmemory controlled by system software. The parameters are different fromone transducer position to the other for the sector-sector scan formatsin the first embodiment of the present invention. The parameters shouldbe down-loaded in real-time for every transducer position. For thesecond embodiment the parameters only need to be loaded once. The 2Dscan converter is for image geometric transformation. The scan converteroutputs images to the Raster Processor (RP) to form the transparentimage and real-time C-scan image, which is then displayed after theVideo Manager. The 2D SC output is also sent to the Image Processor (IP)and through which to the IM. Once every volume scan (the transducermoves from one end to the other), the IM sends image data to the SC toform three orthogonal images x(i), y(j) and z(k) shown in FIG. 1. Theimages are displayed in the other three quadrants of the screen. Theseimages are updated for every volume scan (1-4 seconds for motor drivenprobe, or longer for manually driven probe).

Three multiplanar images x(i), y(j) and z(k) are orthogonal. In FIG. 1,the image x(i) is the C-scan image. The image y(j), which is orthogonalto the image x(i), is in the transducer elevation direction. The imagez(k) is orthogonal to x(i) and y(j) at a constant distance from thetransducer surface.

The transparent image is defined as the spatial projected compounding ofthe multiplanar images. The projection direction is defined as thenormal direction of the multiplanar image.

The 3D SC consists of three parts, a 2D Scan Converter (2D SC), an ImageProcessor (IP) and an Image Memory (IM) in an ultrasound imaging system.To simplify the conversion process, the 3D SC uses multiple 2D scanconversions to complete the 3D scan conversion. In a first embodiment ofthe present invention, a 3D SC apparatus is provided which performsreal-time C-scan and transparent imaging in addition to the regular 2DB-scan imaging. The interpolation quality is slightly lower than that ofthe second embodiment herein. Also, some special requirements on theimage acquisition control in the elevation direction are needed.

In the first embodiment, the scan conversion process has two phases. Inthe first phase, the 2D SC converts the input 2D B-scan image fromsector format to rectangular format. The output image is the projectedview of the input image on the center plane in the transducer elevationdirection. The output image is stored in Image Memory (IM) and the ImageProcessor (IP). In the second phase, the 2D SC reads the stored imagedata in the C-scan plane and converts the image from one rectangularformat to another rectangular format which has the correct shape andsize in the 3D space. The output image from the second conversion isthen stored back to the IM. All image interpolations in both scanconversions are limited in the 2D plane to simplify the process. Afterone sector-to-rectangular and another rectangular-to-rectangular scanconversion, the image data inside the IM has been converted fromsector-sector format to raster cubic format. The converted data matrixis used to form multiplanar, oblique and volumetric images. A criticalstep to simplify the scan conversion of the second phase is to acquirethe input 2D slices in a manner which allows the parallel intersectionlines of the 2D slices to have equal space on the C-scan plane. In thiscase, the 2D slices are acquired not in equally spaced angularincrements but in equally spaced tangent angular increments duringscanning in the transducer elevation direction φ. One or multiplereal-time C-scan images are formed inside the IP. One real-timetransparent image is formed with the assistance of the Raster Processor(RP).

In a second embodiment of the present invention, a 3D SC apparatus isdisclosed which does the conversion only after the scan. The image datais saved in the IM during the scan. The interpolation quality is betterin this embodiment than in the first embodiment disclosed. Also theimage acquisition control is simpler than that of the first embodiment.

In the second embodiment of the present invention, during imageacquisition, the 2D SC first converts the input 2D slices from sector torectangular format. The SC output is stored back to IM through IP. Onlyregular real-time B-scan images are displayed. The transducer scans inits elevation direction φ in equally spaced angular increments. Afterimage acquisition, the 2D SC reads the image data out of IM in thevertical plane which is perpendicular to the B-scan slices. The 2D SCconverts the image data from sector to rectangular format and then sendsit back to IM. After two sector-to-rectangular conversions, the imagedata in IM has beep converted to a cubic matrix. Multiplanar, oblique,and other images can be formed based on the data matrix.

FIG. 1 shows the sector-sector scan format, where

r(l): the radius distance of the l^(th), sample on an acoustic vector,r(l)=0, l:l=1,2, . . . L, where L is the total number of samples on avector;

φ(m): the angle between the m^(th) vector and the center vector on thesame scan plane, the total number of vectors on a plane is 2M+1Θ(M)=0,Θ(m)=-Θ(2M+1-m), where m:m=1,2, . . . 2M+1;

φ(n): the angle between the n^(th) scan plane and the center plane inthe transducer elevation scan direction, the total number of planes inthe 3D scan volume is 2N+1, φ(N)=0, φ(n)=-φ(2N+1-n), where n:n=1,2, . .. 2N +1.

The sample value at 3D space location (l,m,n) is s(l,m,n). The sampledata is stored vector by vector in the IM shown in FIG. 2.

The 3D SC converts the sample data set to a cubic voxel matrix shown inFIG. 3. To simplify the discussion, the following definitions are usedin reference to FIG. 1 and FIG. 3;

x(i): the distance from the apex of the scan volume (l=1) to a planewhich has vector (l,M,N) as its normal vector. x(1)=0, and i:i=1 to I. Iis the total number of planes from top to bottom of the cubic volumeshown in FIG. 3;

y(j): the distance from the apex to a plane which is parallel to theplane defined by (l,M,n), for any l and n. y(J)=0, y(J)=-y(2J+1-j), andj:j=1 to 2J+1. In the cubic voltime in FIG. 3, the total number ofplanes from left to right is 2J+1.

z(k): the distance from the apex to a plane which is parallel to theplane defined by (l,m,N), for any l arid m. z(K)=0, z(k)=-z(2K+1-k), andk:k=1 to -2K+1. In the cubic volume in FIG. 3, the total number ofplanes from front to back is 2K+1.

Each 3D voxel location is represented as (i,j,k), and the voxel value isv(i,j,k).

As stated above, there are three pieces of hardware involved in the 3DSC: 2D SC, IM and IP. A known programmable 2D SC is fully capable ofhandling the necessary processing.

To simplify the discussion of the Image Memory (IM), assume that the IMhas a cubic shape structure as shown in FIGS. 2 and 3. If the smallestaccess unit of the IM is a vector, this vector should have, at any time,one of the three directions as defined in FIGS. 2 and 3. For example, inregular 2D mode, the vector in IM is defined in the r direction in FIG.2 or the x direction in FIG. 3. The other two directions are controlledby vector number and frame number in the image transfer header file. In3D mode, besides this definition, under system CPU control, the IM isable to consider the vector in both Θ and φ directions. If the vector isdefined in the Θ direction, the vector number and the frame,number inthe header file control the φ direction and the r direction. Undersystem software control, an address multiplexer in the IM can switch thedirection definition. Further, with respect to the IP necessary for 3Dscan conversion, only a few image frame buffers are required inside theIP. In addition to the stated three pieces of hardware (2D SC, IM, andIP), the system CPU controls several look-up tables (LUTs) which holdthe scan conversion parameters.

First Embodiment

FIG. 8 shows a flowchart of the first embodiment of the 3D scanconverter design of the present invention.

Image Acquisition Control

A regular phased array or tightly-curved array is driven by a motor,moving in the transducer elevation direction (φ direction). A positionencoder senses the movement and provides a trigger signal for the imageacquisition. The 2D images are acquired based on the following rule:

    tan[φ(n)]-tan[φ(n-1)]=C

where C is a constant for all n's. This rule provides constant intervalsbetween the scan planes on a horizontal plane; see FIG. 4.

First 2D Scan Conversion

To simplify the discussion, we make following assumption: the parameterX_(Start) =0 in the SC Polar to Rectangular Conversion, so that the apexof the scan is at the top of the rectangle. All other parameters are thesame for all scan frames (n:n=1,2 . . . 2N+1), except parameterX_(IncrL), which, as shown in FIG. 5, is the following:

    X.sub.IncrL (n)=X.sub.IncrL (M)/cos[Θ(n)], for (n:n=1,2 . . . 2N+1).

The system software downloads the parameter X_(IncrL) (n) from a LUT toa 2D SC for every frame of acquired images as shown in path [1] in FIG.8.

During real-time image acquisition, the image data goes to a 2D SC forreal-time scan conversion, reference number [2] in FIG. 8. The output of2D SC is received by RP and IP, reference number [3] in FIG. 8. Adynamic transparent image is formed (compounded) inside the RP, andsends it to VM for image display at reference number [3a] in FIG. 8. Oneor more C-scan images can be formed in the IP.

The IP also sends the post-scan-converted images back to the IM,reference number [4] in FIG. 8. This occurs between two real-time scanconversions. The n^(th) frame image is stored in the n^(th) layer of theIM (k=n) as shown in FIG. 3, where the total number of layers 2K+1 islarger than the total number of 2D slices in the scanned volume, 2N+1.

Real-Time C-Scan Image Formation

During real-time image acquisition and display, the user can specify oneor more C-scan image locations. C-scan images are formed in real-time.To simplify the discussion, assume the user wants to display two C-scanimages coming from depth x(i) and depth x(i+D), shown in FIG. 7, where Dis a constant, and x(D) is the distance between the two images.

When receiving the n^(th) frame real-time image from a 2D SC, the IPputs the line i(LN=i) and line i+D (LN=i+D) into the n^(th) row of twoimage buffers, Buffer 1 and Buffer 2 (see FIG. 6). At the end of onevolumetric scan, the IP sends the two images in the two buffers oneafter the other to the 2D SC input, reference number [5] in FIG. 8. Theoutput order of the two images from IP is column by column (jdirection). The 2D SC performs two rectangular-to-rectangular scanconversions and sends its output to display, reference number [6] inFIG. 8. The scan conversion parameters are assigned the following rules:

No change in the Y_(scale) ;

The X_(scale) (actually in the z direction in FIG. 3) is increased (thedisplayed image size in this direction is reduced) to make the imagescale in y and z directions proportional. The increasing factor isproportional to the ratio of 1/i (or 1/(i+D));

The center of the converted image in the X direction (z direction inFIG. 3) is aligned to the line k=K in FIG. 3.

After the rectangular-rectangular scan conversion, the SC outputs thetwo C-scan images to RP and the VM for display.

Second 2D Scan Conversion and 3D Cubic Image Data Set Formation

After finishing real-time image acquisition, the user may decide toreview the 3D volume image. In order to easily form multiplanar,oblique-scan, and volumetric images, the image data in IM is convertedto a cubic data matrix. The image data in IM is sent to the 2D SC andback to IM again to complete the process.

The IM outputs its data "vector" in the z direction (see FIG. 3) to the2D SC, reference number [7] in FIG. 8. Here k is the Range address, andj is the Vector address. The image data in the top-down layer i in the xdirection is the i^(th) frame image.

The i^(th) frame image is sent to the 2D SC for arectangular-rectangular scan conversion which is the same as theconversion in the Real-Time C-scan Image Formation discussed above. Theoutput of the SC is fed back through IP to IM into the i^(th) layer,reference number [8] in FIG. 8. The new data overwrites the old data inthe same layer in IM. The IM contents are completely updated after iruns from 1 to I. The image data in IM now is arranged in cubic shape asdepicted in FIG. 3.

Second Embodiment

In the first embodiment discussed above, the second scan conversion isnot in the r-φ direction, but the z direction. This could introduceimage point-spread function distortion similar to the distortion in somesimple 2D scan converters which use one-dimensional linearinterpolation. The second embodiment overcomes this problem, however,this embodiment cannot provide C-scan images and dynamic transparentimages in real-time.

FIG. 9 shows a flowchart of the second embodiment of the 3D scanconverter design of the present invention. The following description ofthe second embodiment of the present invention makes reference to FIG. 9and the reference numbers therein.

Image Acquisition Control

A regular phased array or tightly-curved array is driven by a motor,warbling in the transducer elevation direction (φ direction). A positionencoder senses the movement and provides a trigger signal for imageacquisition. 2D images are acquired based on the following rule:

    φ(n)-φ(n-1)=C,

where C is a constant for all n's. This rule provides constant angularintervals between the scan planes. PG,17

First 2D Scan Conversion

All SC parameters are the same for all scan frames (n:n=1 . . . 2N+1),path [1] of FIG. 9. During real-time image acquisition, the image datagoes to 2D SC for real-time polar-to-rectangular scan conversion,reference number [2] in FIG. 9. The output of 2D SC is received by IP,reference number [3] in FIG. 9. The IP input data vector is in ydirection. The IP sends the post scan-converted images back to IM,reference number [4] in FIG. 9. The IP output vector is in the Xdirection. This occurs between two real-time scan conversions (if thereis some time left over). The n^(th) frame image is stored in the n^(th)layer of the IM (k=n) as shown in FIG. 3, where the total number oflayers 2K+1 is larger than the total number of 2D slices in the scannedvolume, 2N+1.

Second 2D Scan Conversion and 3D Cubic Image Data Set Formation

After real-time Image acquisition, the IM outputs its dam "vector" inthe vertical x direction (see FIG. 3) to the 2D SC, reference number [5]in FIG. 9. Here i is considered as the Range address, and k isconsidered as the Vector address. The image data in the left-right layerj in the y direction is considered as the j^(th) frame image. The j^(th)frame image is sent to the 2D SC for a polar-rectangular ("r"-φ to x-z)scan conversion. Here "r" is the IM output vertical vector. The SCparameters are the same for all the frames. The output of the SC is fedback through IP to IM into the j^(th) layer, reference number [6]. Thenew data overwrites the old data in the same j^(th) layer in IM. The IMcontents are completely updated after j runs from 1 to 2J+1. The imagedata in IM is now arranged in cubic shape as shown in FIG. 3.

Although other modifications and changes may be suggested by thoseskilled in the art, it is the intention of the inventor to embody withinthe patent warranted hereon all changes and modifications as reasonablyand properly come within the scope of his contribution to the art.

I claim:
 1. A three-dimensional scan converter system for generatingreal-time orthogonal multiplanar and transparent images in addition totwo-dimensional B-scan images in an ultrasound imaging system,comprising:means for ultrasonically scanning a subject to obtainreal-time image data representing an image of said subject, saidultrasonic scanning means further comprises transducers and athree-dimensional position control means having a position encoder forindicating current transducer position; image memory means for storingimage data representing an image of a subject; two-dimensional scanconverter means for converting the stored image data intothree-dimensionally formatted stored image data in real-time; imageprocessing means for operating on said three-dimensionally formattedstored image data for successive display of said multiplanar orthogonalimages; system software and a CPU, having one or more look-up tablescontaining scan converter parameters corresponding to said transducerposition therein, for downloading said parameters in real-time to saidscan converter means; a raster processing means for compounding saidtransparent image; and display means for displaying saidthree-dimensional images.
 2. A method for generating real-timeorthogonal multiplanar and transparent images in addition totwo-dimensional B-scan images in an ultrasound imaging system havingtransducers, comprising the steps of:ultrasonically scanning with equalintervals between scan planes a subject to obtain two-dimensional imagedata representing an image of said subject; storing said two-dimensionalimage data; indicating position of said transducers; downloading scanconverter parameters corresponding to a position of said transducersfrom a look-up table into a scan converter; performing a firsttwo-dimensional scan conversion on said stored image data in a firstdirection into raster two-dimensional images with controlled linedensity; storing said resulting first two-dimensionally scan convertedimage data in said image memory; performing a second two-dimensionalscan conversion on said stored resulting first two-dimensionally scanconverted image data in a second direction orthogonal to said firstdirection; storing said resulting second two-dimensionally scanconverted image data in raster cubic format in said image memory;displaying stored three-dimensional image data.
 3. The method of claim2, wherein said ultrasonic scanning step of acquiring two-dimensionalimage data has constant intervals between scan planes defined bytan[φ(n)]-tan[φ(n-l)]=C, where C is a constant for all integer values ofn.
 4. The method of claim 2, wherein said ultrasonic scanning step ofacquiring two-dimensional image data has constant intervals between scanplanes defined by φ(n)-φ(n-1)=C, where C is a constant for all integervalues of n.
 5. The method of claim 2, wherein said controlled linedensity of said raster two-dimensional images after said first scanconversion step is further defined by X_(IncrL) (n)=X_(IncrL)(M)/cos[φ(n)].
 6. The method of claim 2, wherein said controlled linedensity of said raster two-dimensional images after said first scanconversion step is further defined by X_(IncrL) (n)=Constant.